Certain patients with malignant lymphomas originating from peripheral T cells develop a rapidly fatal syndrome which mimics malignant histiocytosis. It is suspected that the pathogenetic mechanism of this phenomenon may involve a lymphokine produced by the neoplastic T cell which can stimulate the phagocytic cells of the reticuloendothelial system. In order to test this hypothesis, neoplastic cells from fresh biopsies of patients with malignant lymphoma are placed in overnight culture, and supernatants are tested for the presence of soluble factors which are able to affect human phagocytic cells in vitro. Normal cells are also assayed for their ability to produce this factor. Il-2 dependent T cell lines have been developed to further characterize the source of lymphokine activity termed PIF or phagocytosis inducing factor. Preliminary biochemical characterizations of PIF has been conducted.